STORY STIFFNESS

I would like to know how staad calculate story stiffness?

I am using FLOOR DIAPHRAGM and PRINT STORY STIFFNESS to get story stiffness from staad.

Now suppose structure with 5 floor and same height. All column and beam are of same size. When I check story stiffness result why they differ floor to floor when all floor are similar?

Kindly reply.

Regards,

PNSTORY STIFFNESS.std

  • The logic behind the calculation of Story Stiffness -
    STAAD identifies column and shear wall (without opening) as vertical component for the purpose of computing lateral stiffness of the story. The vertical stiffness of a column is calculated as 12EI / L3 where E is the Young’s modulus, I is the moment of inertia and L is the length of the column respectively and that for a shear wall (without opening) is calculated as Ph3/3EI + 1.2Ph/AG (i.e., summation of flexural stiffness and shear stiffness, obtained as deflection of a cantilever wall under a single lateral load P at its top) where h is the height, A is the cross-sectional area and G is the shear modulus of the wall (E and I carry usual meaning). The summation of lateral stiffness of all columns and shear walls at a particular floor level constitute the total lateral stiffness of that particular story or floor level.



  • Please find the attached document which explains the process of calculation of storey stiffness in the presence of rigid floor diaphragm.



  • Thank you for your kind reply.
    In attached file structure has 5 floor. Each floor have 3m height. All column and beam have same property.
    I suppose to get same story stiffness from staad. But result are giving varying stiffness floor wise. Why is it so?
    Kindly go through attached file.
    Regards,
    PN
  • I mean to say if we use equation 12EI / L3. It should result in same story stiffness for all floors.
    Regards,
    PN
  • Please refer to the document that I have attached. Storey stiffness is calculated from the relative displacements of the centre of mass of each floor. A sample calculation has also been explained.